1. Field of the Invention
The invention relates to surgical systems and methods and, more particularly, to a system and method for facilitating hip surgery.
2. Background Information
Hip arthritis due to malformations, fractures and diseases constitutes a major source of disability. Reconstruction of the hip, whether involving first time hip replacement or replacement of failed components, is complicated by the great mobility of the hip joint and the uncertainty in the exact position of pelvis and femur during surgery. When a hip joint is replaced, as in total hip arthroplasty or hip resurfacing arthroplasty, critical factors to assess include changes in leg length, offset, and anterior-posterior position. Leg length refers to the longitudinal extent of the leg, and may be measured, e.g., from a location on the pelvis down to some location along the leg, such as a point of the femur. Offset refers to the lateral dimension through the hip. This lateral dimension having contributions from changes during the reconstruction on both the pelvic and the femoral sides. Further, changes in anterior-posterior (AP) position of the joint may be measured along a third orthogonal axis.
Changes in leg length, offset and AP position following surgery can lead to a loose and unstable joint, exposing the patient to a risk of dislocation and revision, i.e., corrective surgery. Unequal leg lengths can also lead to trunkal imbalance, excessively tight tissues, and discomfort.
Typically, changes in leg length, offset and AP position are checked during total hip arthroplasty, for example, following a “trial reduction”. That is, a trial stem is driven into the femur, a trial head is attached to the stem, and a trial acetabular liner is typically inserted into the prosthetic metal acetabular cup that has been inserted into the socket of the pelvis. The femoral trial components are then “reduced” into the acetabular components, i.e., temporarily assembled together. During this trial reduction, critical parameters, such as leg length, offset, AP position, the hip's free range of motion and its stability, are assessed to determine if the goals of the reconstruction are being achieved. If so, the trial prosthesis components are then replaced with the final prosthesis components.
Several systems and methods currently exist to determine changes in leg length, offset and AP position. For example, rulers and gauges exist to physically measure changes in leg length in particular. Such physical measurements, however, are often inaccurate. Another method is to use preoperative images and to plan component selection and placement to optimize leg length and offset. During surgery, the components are placed so as to be as close as possible to the pre-operative plan. Other techniques involve the taking of images during the operation, such as X-rays, but these images are typically of unknown magnification, may not have been taken in the correct plane, and often do not show both legs on the same image, limiting the surgeon's ability to compare one side to the other.
Further attempts at measuring changes in leg length, offset, and AP position have involved computer-assisted surgical navigation techniques. Surgical navigation currently involves tracking the positions of the bones using optical or electromagnetic tracking systems. Typically, position tracking frames or devices are affixed to the bone to be tracked and then a coordinate system for that bone is established so that the tracking frames then track the coordinate system itself. The coordinate system of the pelvis is typically determined by finding three landmarks. These are the two anterior superior iliac spines and the pubic symphysis.
Progression of the art has stalled though because proper establishment of a femoral coordinate system requires calculation of the center of rotation of the hip. U.S. Pat. No. 6,711,431, for example, describes a non-imaging navigation system for hip replacement surgery. With this system, a center of rotation of the hip is found before reconstruction is performed in order to determine leg length and offset. After the trial prosthetic components are installed, the system requires that the new center of rotation of the hip be determined in order to calculate a change in leg length and a change in offset. Unfortunately, the hip joint that is being reconstructed is often so destroyed that such a calculation is either inaccurate or impossible to determine. It can also be time-consuming and difficult.
One attempt to circumvent this problem has been to calculate only the pelvic coordinate system and then to simply digitize a landmark somewhere on the femur that would be considered a starting point before reconstruction. The theory is that if a temporary reconstruction is performed and the same landmark is digitized, then changes in leg length, for example, could be calculated. This method, however, is imprecise due to the difficulty in bringing the leg back into the exact same position that it was in before, thereby causing the calculation to give very inaccurate information.
Accordingly, a need exists for an efficient and simplified system and method for determining changes in leg length, offset and AP position during hip surgery.